With increasing use of laser technology in various applications, the occurrence of speckle effect has increased. The speckle effect refers to a pattern in which there is a random distribution of varying light intensity levels and may occur with coherent light sources. In the case of a coherent light source, such as laser light, a speckle pattern of bright and dark spots appears in the area being lit. It is caused by the interference of many waves of the same frequency, but different phases and amplitudes, combining and resulting is a wave whose amplitude and intensity varies randomly. This occurs when coherent light (light consisting of only one wavelength) is scattered off a rough surface. It should be noted that “rough” refers to the surface texture at a microscopic level. Thus, in minimally invasive surgical (MIS) procedures or applications, all surfaces can be considered rough.
When a coherent light source, such as laser light, is used as the light source for minimally invasive surgical (MIS) or endoscopic procedures, speckle is an undesirable effect as it makes the image appear grainy to a viewer. Conventional light sources for MIS and endoscopy (such as metal halide bulbs, halogen bulbs, xenon bulbs, and light emitting diodes (LEDs)) do not have a speckle pattern because the light has low coherence. Speckle patterns are generally not noticeable from light with low coherence because the amplitude variations tend to average each other out. Despite the speckle pattern, a laser light source may have benefits over conventional light sources, including power efficiency, cost, low heat generation, small size, color pulsing, narrow-band imaging, and the ability to generate controlled light outside the visible spectrum (infrared or ultraviolet). Therefore, a coherent light source system, such as a laser light source system, without the speckle pattern is desirable.
Others have attempted to address the speckle effect with laser light using a variety of methods, each of which essentially lowers the coherence of the light. One of the simplest methods is to place a moving diffuser in the light path. The diffuser can rotate or oscillate and this movement lowers the light coherence. Other methods include systems of mirrors, homogenizing lenses, and/or light guides. A common application of the technology is in laser projectors.